1. Field of the Invention
This invention relates to semiconductor devices and, more particularly, to field effect transistors (FETs).
2. Art Background
The use of gallium arsenide field effect transistors (GaAs FETs) in microwave circuitry although recently initiated has rapidly increased. These FETs which include a drain, a source and a gate to control current flow between the source and drain, presently are contemplated for use in among other applications the microwave processing circuitry of telecommunication satellites. Obviously for applications such as satellite circuitry it is necessary that the components employed have an extremely long operating lifetime.
Precautions are now being taken in the production of GaAs FETs to enhance device lifetime. For example, the use of a ledge configuration for the contact pad of a GaAs FET has been found useful for enhancing lifetime. (See U.S. Pat. No. 4,196,439, issued Apr. 1, 1980.) Further lifetime enhancement is achieved by limiting dendrite formation during production of the drain and source contacts. In this technique when a metallization layer is alloyed into the GaAs pad to form an electrical contact, the composition of the metallization layers and the conditions used during alloying are carefully controlled to prevent extension of dendrites into the GaAs active region. (See U.S. Pat. No. 4,301,188 Nov. 17, 1981, by William C. Niehaus, that is hereby incorporated by reference.) An inert layer deposited on the exposed surface of the finished device to form a passivation region is also generally useful in semiconductor devices to increase production yield. A passivation region is employed to protect the surface and to avoid the introduction of impurities from the ambient into the active regions of the device. Although the previously described techniques have shown promise for improving device lifetimes, these devices are as yet not completely satisfactory for demanding applications such as required in telecommunication satellites.